mm/damon/tests/core-kunit: remove unnecessary damon_ctx variable on damon_test_split_at()

mm/damon/tests/core-kunit: remove unnecessary damon_ctx variable on damon_test_split_at()

damon_test_split_at() dynamically allocates a 'struct damon_ctx' object,
but it is not really being used in the code other than handling the
allocation failure and deallocating it at the end of the function.  Remove
the unnecessary allocation and deallocation of the object.

Link: https://lkml.kernel.org/r/20251101182021.74868-22-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: David Gow <davidgow@google.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>